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TRANSCRIPT
REVIEW
Synthesis of Photoactive Materials by Sonication:Application in Photocatalysis and Solar Cells
Juan C. Colmenares1• Ewelina Kuna1
•
Paweł Lisowski1
Received: 3 June 2016 / Accepted: 30 July 2016 / Published online: 10 August 2016
� The Author(s) 2016. This article is published with open access at Springerlink.com
Abstract In recent years, a good number of methods have become available for the
preparation of an important group of photoactive materials for applications in
photocatalysis and solar cells. Nevertheless, the benefits derived from preparing
those materials through unconventional approaches are very attractive from the
green chemistry point of view. This critical review work is focused on sonication as
one of these promising new synthetic procedures that allow control over size,
morphology, nanostructure and tuning of catalytic properties. Ultrasound-based
procedures offer a facile, versatile synthetic tool for the preparation of light-acti-
vated materials often inaccessible through conventional methods.
Keywords Photoactive materials by sonication � Photocatalysis � Ultrasounds �Solar cells � Perovskites � Quantum dots
AbbreviationsUV Ultraviolet
BFO Bismuth ferrite
SPD Sonophotodeposition
RT Room temperature
RB5 Reactive black 5
SnS Stannous sulfide
VOCs Volatile organic compounds
This article is part of the Topical Collection ‘‘Sonochemistry: From basic principles to innovative
applications’’; edited by Juan Carlos Colmenares Q., Gregory Chatel.
& Juan C. Colmenares
1 Institute of Physical Chemistry of the Polish Academy of Sciences (PAS), Kasprzaka 44/52,
01-224 Warsaw, Poland
123
Top Curr Chem (Z) (2016) 374:59
DOI 10.1007/s41061-016-0062-y
USP Ultrasonic spray pyrolysis
ESR Electron spin resonance spectroscopy
QDSCs Quantum dot sensitized solar cells
PSCs Perovskite solar cells
PCE Power conversion efficiency
DSSCs Dye sensitized solar cells
CdTe Cadmium telluride
NPs Nanoparticles
QDs Quantum dots
ROS Reactive oxygen species
HSs Hierarchical structures
UST Ultrasound treatment technique
SVADC Substrate vibration-assisted drop casting method
ITO Indium tin oxide
spiro-OMeTAD N2,N2,N2
0,N2
0,N7,N7,N7
0,N7
0-octakis(4-methoxyphenyl)-9,9
0-
spirobi[9H-fluorene]-2,20,7,7
0-tetramine
1 Sonochemical Preparation of Photoactive Catalytic Materials
The phenomenology of fabrication of highly efficient photocatalysts through an
ultrasound route represents a very interesting and important area in science and
technology, and it holds great potential for photocatalysts preparation in the near
future [1–3]. In comparison with traditional sources of energy, ultrasound ensures
unusual reaction conditions in liquid phase reactions due to the cavitation
phenomenon (extremely high temperatures and pressures are formed in very short
times in liquids) [3, 4]. Furthermore, application of ultrasound irradiation to a
photocatalytic system might enhance both the bulk and localized mass transport and
consequently expedite the molecular contact and the photocatalytic activity
[1, 2, 4–6]. A considerable number of novel photocatalysts with novel nano-/
microstructures (e.g., ZnWO4 [7], CdMoO4 [8], V2O5 [9], CuMoO4 [10], TiO2/
Bi2O3 [11]) have been prepared by means of ultrasound-assisted methodology. In
this subsection, we would like to focus the readers’ attention on ultrasound-
promoted procedures for the preparation of photocatalysts, and the potential
application of these materials in the photocatalytic degradation and selective
oxidation of organic compounds.
1.1 Sonochemical Synthesis of TiO2 Photocatalyst
The current status of outstanding progress made by titanium dioxide (TiO2), as the
most widely used oxide semiconductor, has attracted considerable interest owing to
its various applications in solar-driven hydrogen production, photocatalytic
decomposition of pollutants, solar cells and so forth [12–14]. It is well known
that overall efficiency for the solar-driven photocatalysis is very limited, because of
its wide bandgap in the UV region, which accounts for less than 5 % of the total
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solar irradiation [15]. Currently, hydrogenated TiO2 nanoparticles (e.g., blue [16],
brown [17] and black [18]) have opened a new avenue to the long-wavelength
optical absorption and greatly enhanced photoactivity of the catalysts. Hydro-
genated TiO2 is intensively investigated due to its improvement in solar absorption,
but there are major issues related to its structural, optical and electronic properties,
and therefore an easily compatible method of preparation is much needed [19].
Recently, disorder-engineered TiO2 nanocrystals treated in a hydrogen atmosphere
under 20 bar of H2 atmospheres and 200 �C for approximately 5 days showed a
colour change to black with a reduction in the bandgap energy up to *1.54 eV
[20]. It should be pointed out that band gap narrowing may be traced back to the
surface disorder without the formation of Ti3? centres. In other words, formation of
surface disorders, oxygen vacancies, Ti3? ions, Ti–OH and Ti–H groups, and band
edge shifting are responsible for the optical properties and photocatalytic activity of
black or hydrogenated TiO2. Interestingly, Ti3? is not responsible for optical
absorption of black TiO2, while there is evidence of mid-gap states above the
valence band edge due to hydrogenated disorders [21–23]. It should be also noted
that hydrogen doping induced a high density of delocalized Ti3d electrons leading
to improved charge transport properties [24]. Furthermore, the hydrogenated black
TiO2 exhibited highly efficient activity for the photocatalytic splitting of water [25].
Moreover, hydrogenation procedures need a high annealing temperature (over
400 �C) or high-pressure (e.g., 20 bar) and H2 is flammable and explosive
[16, 17, 23–26]. The first attempt to prepare amorphous hydroxylated TiO2 with
various degrees of blackness was based on the ultrasonic irradiation of high power
intensity with enhanced photocatalytic activity of acid fuchsin degradation [27]
(Table 1, Entry 1). A key feature of this system is that power density of employed
ultrasonic irradiation was as high as 15 W mL-1, and a low ultrasonic power
density could not make such changes in colour. Furthermore, ultrasound was
employed to modify the original TiO2, which prepared amorphous hydroxylated
TiO2 with black appearance, large surface area (328.55 m2 g-1) and enhanced
photocatalytic methylene blue decomposition. It was found out that ultrasonic
irradiation could accelerate the hydrolysis of TiO2 and reduce its particles size and
form amorphous hydroxylated TiO2 by long time ultrasonication, giving rise a
material with higher absorbance intensity through the whole visible light and near-
infrared regions.
Worth mentioning is the work by Mao et al. [28] on the preparation of
mesoporous titanium dioxide (TiO2) inside the periodic macropores of diatom
frustules by sonochemical condensation of titania precursor, and then thermal
treated at elevated temperatures, resulting in hierarchical macro/mesoporous
materials. Furthermore, hierarchical structure provides a large number of accessible
active sites for efficient transportation of guest species to framework binding sites. It
was found that a high photocatalytic degradation of methylene blue as compared
with P25 can be achieved with the composite having a certain loading amount of
TiO2 (30 wt%), attributing to its hierarchical macro/mesoporous structures. It
should be also noted that mesoporous titanium TiO2 with wormhole channel
together with interconnected 3D structures inside diatom pores prepared by
sonochemical synthesis with organic surfactant as structure directing agent has
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potentially important structural features for photocatalytic reactivity, because
channel branching within the framework may facilitate access to reactive sites on
the framework wall, which is most critical for its use as photocatalyst.
In terms of improvement of TiO2 photocatalytic properties under solar light by
means of a suitable modification, enhanced photocatalytic activity of TiO2
nanocomposites for the degradation of dyes and Gram negative bacteria
(Escherichia coli) was obtained by doped TiO2 with Al2O3, Bi2O3, CuO and
ZrO2 [29] and synthesized via sonochemical method. It is interesting to notice that
Table 1 Selected research studies of ultrasound assisted processes in the synthesis of photocatalysts and
photoactive catalytic materials
Entry Photocatalyst Ultrasound
source
Reaction conditions Remarks Refs.
1 Black TiO2 15 W mL-1 0.5, 1, 2, 4 and 8 h at
80 �C, 100 mL
Totally disorder structure of
amorphous TiO2 both before
and after ultrasonic treatment
was observed
[27]
2 CaTiO3,
BaTiO3,
SrTiO3
45 kHz,
60 W
Ambient conditions
for 10 h, glass tube
sealed with a screw
cap
CaTiO3 and BaTiO3
nanoparticles with almost
regular spherical shape and
uniform particle size
(*20 nm) were observed.
SrTiO3 particles were found to
agglomerate more strongly
leading to cubic-like
aggregates with edge lengths
varying (100–300 nm)
[31]
3 ZnO/
Ag3VO4
12 mm
diameter
Ti horn,
75 W,
20 kHz
1, 1.5, 2, 3, and
4 h at RT, 150 mL
The nanocomposites prepared
by 2 h ultrasonic irradiation
have the best activity
[40]
4 ZnO/AgI/
Fe3O4
12 mm
diameter
Ti horn,
75 W,
20 kHz
0.5, 1, 2, and 4 h
at RT, 150 mL
Photocatalyst prepared by
ultrasonic irradiation for
1 hour has superior activity
compared to other samples,
and has remarkable stability
and excellent magnetic
filtration from the treated
solutions
[41]
5 ZnO/AgI/
Ag2CrO4
12 mm
diameter
Ti horn,
75 W,
20 kHz
0.25, 0.5, 1, 2, and
3 h at RT, 150 mL
Material prepared by ultrasonic
irradiation for 1 h has the
superior activity
[42]
6 g-C3N4 12 mm,
33 Hz,
150 W
5 h at ambient
temperature, 150 mL
g-C3N4 sheet possesses porous
structure with high surface
area and large pore volume
[43]
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Bi2O3–TiO2 nanocomposite is more efficient toward the degradation of the dyes
under solar light and TiO2 is the least efficient photocatalyst; the efficiency is in the
order: Bi2O3–TiO2[Al2O3–TiO2[CuO–TiO2[ZrO2–TiO2[TiO2. It should
be also noted that the antibacterial activity of all the five synthesized nanocom-
posites was tested against Gram negative bacteria (E. coli) by varying the
concentrations (250, 500, 750 and 1000 lg). Among them, Al2O3–TiO2 shows some
significant zone of inhibition around the film under dark condition. Recently, certain
novel promising photocatalysts prepared by ultrasound-assisted method have been
described in several research papers and review articles [1–3].
1.2 Sonochemical Synthesis of Selected Photocatalysts
Sonochemical method has been used extensively to generate novel materials with
unusual (photo)-catalytic properties, due to its unique reaction effects. For example,
addition of titanium iso-propoxide induces the generation of new linear polymeric
chains type nickel-titanium-ethylene glycol (Ni–Ti–EG), as revealed by the color
change (from green to light blue). This light blue polymer coagulates to form a
uniform rod-like precursor by van der Waals interactions [30]. Another elegant
strategy to prepare ternary CaTiO3, SrTiO3 and BaTiO3 materials by a one-step
room-temperature ultrasound synthesis in ionic liquid for photocatalytic applica-
tions (photocatalytic hydrogen evolution and methylene blue degradation) was
recently described [31] (Table 1, Entry 2). It is also important to note that the use of
ionic liquids in sonochemistry is advantageous due to their low measurable vapor
pressure, low viscosity, low thermal conductivity, and high chemical stability,
which are all significant parameters to generate efficient cavitation.
Photoactive materials such as SrZrO3have been recently explored because they
exhibit suitable properties for photocatalytic hydrogen evolution from water
splitting [32]. The results confirm that SrZrO3 was stable with time of hydrogen
evolution and a very high rate (35 lmol g-1 h-1) was exhibited by SrZrO3 prepared
by ultrasound-assisted synthesis. Recently, multiferroics such as BiFeO3 (BFO) are
worthy of notice due to their unique and strong coupling of electric, magnetic, and
structural order parameters in terms of practical applications, since both ferroelec-
tric and antiferromagnetic ordering temperatures are well above room temperature
(Curie temperature 830 �C and Neel temperature 370 �C) [33]. Moreover, other
researchers have applied BFO materials prepared by sono-synthesis with high
photocatalytic activity under solar light in degradation of various compounds such
as Rhodamine B [34], methylene blue [35], Reactive Black 5 (RB5) [36] and
phenolic compounds [37]. As anticipated above, BiFeO3 nanoparticles with
measured band gap of 2.0 eV synthesized with ultrasound exhibited higher
crystallization, smaller crystallite size and higher photocatalytic activity than the
material synthesized with conventional methods.
In order to improve the photocatalytic activity of a semiconductor and exploit its
advantageous properties, Fe3O4/ZnO magnetic nanocomposites were synthesized
via a surfactant-free sonochemical method in an aqueous solution (Fig. 1) and
applied in the photo-catalytic degradation of various organic and azo dyes aqueous
solution under UV light irradiation [38]. Surprisingly, these results showed that OH
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radicals generated by ultrasonic treatment were responsible for inducing the
oxidation of Fe2? into Fe3?. As soon as the required hydroxyl groups were available
in the solution, the already-produced Fe3? became saturated, and increasing the
ultrasonic time did not help the dispersion of the formed nuclei. Additionally,
prolongation of the reaction up to 30 min caused oxidation of most of the Fe2?–
Fe3? and as a result, brown FeOOH nanoparticles which are more likely to
agglomerate were also obtained. Moreover, it was shown that ultrasonic power
(100 W) had a significant but rather unpredictable influence on the mean magnetite
particle size (agglomeration size decreased to 100–500 nm). Nanocomposites with
10 % of ZnO exhibited a super-paramagnetic behavior.
Previous research [39] showed that orthorhombic stannous sulfide (SnS)
nanoparticles synthesized via a sonochemical route by 20 kHz sonication had
smaller crystalline size (4 ± 1 nm) in comparison to the nanoparticles that were
synthesized by 50 kHz sonication (6 ± 1 nm). An optical investigation confirmed
that the SnS particles had strong emission bands located at the UV and visible
regions, suggesting that they have the potential to be used as optical devices.
It is known that n–n heterojunctions between two n-type semiconductors can
effectively promote separation of photogenerated electron–hole pairs, due to
formation of internal electric field. Hence, preparation of ZnO/Ag3VO4 nanocom-
posites prepared by an ultrasonic-assisted one-pot method led to the enhanced
photocatalytic activity for organic pollutants degradation under visible-light
irradiation [40] (Table 1, Entry 3). Interestingly, photocatalytic activity of ZnO/
Fig. 1 a Fe3O4 nanoparticles,b Fe3O4–ZnO nanocompositeprepared by surfactant-freesonochemical method.Reproduced from Ref. [38] withkind permission from SpringerScience and Business Media
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Ag3VO4 nanocomposite under visible-light irradiation is about 21, 56, and 2.8-fold
higher than that of the ZnO sample in degradation of rhodamine B, methylene blue,
and methyl orange, respectively. Furthermore, it was revealed that the photocat-
alytic activity was attributed to greater generation of electron–hole pairs due to
photosensitizing role of Ag3VO4 under visible-light irradiation and the efficient
separation of the photogenerated electron–hole pairs due to formation of n–n
heterojunction between the counterparts. Previous studies [41] (Table 1, Entry 4)
showed that photocatalytic activity of the ZnO/AgI/Fe3O4 nanocomposite in
degradation of rhodamine B, methylene blue, and methyl orange is about 32-fold
higher than that of the ZnO/Fe3O4 sample prepared by ultrasonic irradiation method.
The highly enhanced activity of novel ternary ZnO/AgI/Fe3O4 magnetic photocat-
alyst was mainly attributed to visible-light harvesting efficiency and decreasing
recombination of the charge carriers as well. It should be noted that ultrasonic
irradiation time and calcination temperature largely affect the photocatalytic
activity. In another investigation [42] (Table 1, Entry 5), it was found that ternary
ZnO/AgI/Ag2CrO4 photocatalyst with 20 % of Ag2CrO4 is active in rhodamine B
degradation, with nearly 167-, 6.5-, and 45-fold higher conversion than that of ZnO,
ZnO/AgI, and ZnO/Ag2CrO4, respectively, materials also prepared by ultrasonic
irradiation method. Furthermore, visible-light activity of ZnO/AgI/Ag2CrO4 is
about 6.5-, 16-, and 33-fold higher than that of the ZnO/AgI, whereas 45-, 22-, and
136-fold higher than that of the ZnO/Ag2CrO4 in degradation of rhodamine B,
methylene blue, and methyl orange, respectively. It should be pointed out that
photocatalyst prepared by ultrasonic irradiation for 60 min has the superior activity.
Recently, metal-free graphitic carbon nitride (g-C3N4) was synthesized via facile
template-free sonochemical route and enhanced photodegradation of rhodamine B
[43] (Table 1, Entry 6). The authors found that mesoporous g-C3N4 (112.4 m2 g-1)
has almost 5.5 times higher photoactivity than that of bulk g-C3N4 (8.4 m2 g-1)
under visible-light irradiation, which is attributed to the much higher specific
surface area, efficient adsorption ability and the unique interfacial mesoporous
structure that can favor the absorption of light and separation of photoinduced
electron–hole pairs more effectively. Additionally, reactive oxidative species
detection studies indicated that the photodegradation of rhodamine B over
mesoporous g-C3N4 under visible-light is mainly via superoxide radicals.
Zhou et al. [44] prepared Zn3V2O7(OH)2(H2O)2 and g-C3N4/Zn3V2O7(OH)2(H2-
O)2 using sonochemical method with high photocatalytic activities in degradation of
methylene blue. Interestingly, the absorption edge of g-C3N4/Zn3V2O7(OH)2(H2O)2
had red shift reaching 450 nm, and a significantly enhanced photocatalytic
performance in degrading methylene blue, which increased to about 5.6 times that
of pure Zn3V2O7(OH)2(H2O)2. During the ultrasound process, the heterojunction
structure of g-C3N4/Zn3V2O7(OH)2(H2O)2 was formed and this special structure
increased the separation efficiency of photogenerated electron–hole pairs, resulting
in the enhancement of photocatalytic performances. Table 1 shows some detailed
information about ultrasound assisted synthesis methods of photocatalysts made by
selected research groups.
Another approach offering prospects for preparation of nanostructured TiO2/
STARBON�-polysaccharide-derived mesoporous materials by means of
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ultrasound-assisted wet impregnation method was investigated [45]. Interestingly,
carboxylic acid groups can be formed on the surface after initial thermal treatment
at 400 �C under an oxygen-deficient atmosphere. Each carboxyl acid group acts as
an individual nucleation site for TiO2 formation, which is possible under hydrolysis
and condensation reactions promoted by sonication. Finally, the hybrid material
(TiO2/STARBON�) is consolidated after thermal treatment at 400 �C. These
conditions preserve pure, highly crystalline anatase phase (ca. 30 nm), leading to a
reduction in the electron–hole recombination rate at the Starbon surface. TiO2
nanoparticles are strongly anchored and have good contact with the STARBON-800
structure (i.e., no leaching after 240 min of photocatalytic degradation of phenol),
believed to enhance the photoelectron conversion (i.e., as compared with Norit and
Graphene oxide supports) of TiO2 by reducing the recombination of photo-
generated electron–hole pairs.
A similar study using a simple and effective ultrasound-assisted wet impregna-
tion method was developed for the preparation of magnetically separable
TiO2/maghemite-silica photo-active nanocomposites tested in the liquid phase
selective oxidation of benzyl alcohol [46]. Interestingly, photocatalytic selectivity in
organic media (90 % in acetonitrile) towards benzaldehyde was achieved at a
benzyl alcohol conversion of ca. 50 %. Solvents played a significant role in the
photo-oxidation process with materials showing very good conversion and
selectivity in acetonitrile but not in aqueous conditions. Additionally, spatially
ordered heterojunction between TiO2 and c-Fe2O3 and a potential co-catalytic
incorporation of Fe3? into the TiO2 structure might significantly increase the
sensitization and decrease the band gap energy of TiO2, effectively improving the
photocatalytic activity and selectivity of these photocatalysts.
In an attempt to better understand the sonophotodeposition method (SPD), the
iron-containing TiO2/zeolite-Y photocatalyst for the selective oxidation of benzyl
alcohol was investigated [47]. Generally, photocatalyst prepared by the sonopho-
todeposition method showed better results, in terms of alcohol conversion and yield
of benzaldehyde, in comparison with the photocatalyst prepared by an ultrasound-
assisted wet impregnation method. Furthermore, sonication probe and sun-imitating
Xenon lamp were tested, and for the first time, a non-noble metal was deposited on
the TiO2 surface by using visible light, but with the help of ultrasound. It is worth
noting that sonophotodeposition method is an innovative, simultaneous combination
of ultrasonication and ultraviolet irradiation, and is highly energy efficient.
Reactions can be carried out at room temperature and atmospheric pressure, very
short reaction times and without using strong chemical reduction agents. Addition-
ally, a considerable number of novel mono and bimetallic photocatalysts (e.g., Pd/
TiO2 [48], Pd-Au/TiO2 [49], Pd-Cu/TiO2 [50]) have been prepared by sonopho-
todeposition methodology for selective oxidation of volatile organic compounds
(VOCs).
1.3 Ultrasonic Spray Pyrolysis for the Preparation of Photocatalysts
Ultrasonic spray pyrolysis (USP) method is a low-cost, continuous operation, and
environmentally benign process with short processing time, which has been used to
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synthesize functional materials [51]. A distinctive feature of the method is that it can
produce porous microspheres of various compositions without template and with
good phase purity, and the morphology can be easily controlled during the process
to ensure homogeneous composition distribution in the spheres [52, 53]. In this
approach of ultrasonic spray pyrolysis, Suslick et al. [54] synthesized BiVO4
powders with particles ranging from thin, hollow and porous shells to ball-in-ball–
type structures. Interestingly, materials prepared by USP are more active for oxygen
evolving photocatalysts under visible-light irradiation (k[ 400 nm) in AgNO3
solution than commercial BiVO4 and WO3 powders, likely due to differences in the
particle morphology. It was found that the increase of photocatalytic activity is
likely due to the short distances electron–hole pairs must move to reach the surface
in order to perform the desired redox reactions. Recently, USP was adapted to
fabricate hierarchical porous ZnWO4 microspheres and evaluated by the degrada-
tion of gaseous NOx under simulated solar light irradiation [55]. It was found that
synthesis temperature (650–750 �C) was a key factor influencing the microstruc-
tures of resulting ZnWO4 samples, eventually affecting their photocatalytic activity,
and which could be explained by improved optical absorption capability, high
specific surface area, and fast separation/diffusion rate of the photogenerated charge
carriers. Additionally, electron spin resonance spectroscopy (ESR) method indicated
that O2�- and �OH radicals function as the major reactive species for NOx
decomposition. Compared to solid spheres, hollow PbWO4 spheres, which combine
the merits of hollow and porous structures, could present improved mass transfer
and high surface areas [56]. Moreover, hollow structured PbWO4 spheres exhibited
superior photocatalytic activity to solid spheres (NO removal rate is 35 ppb min-1),
due to the differences in microstructure and morphology.
Worth mentioning is the work by Tavares et al. [57] on the enhancement of the
photocatalytic efficiency for the photodegradation of methylene blue applying the
white emission of CaIn2O4 nanocrystals (band gap energy 3.83 eV) prepared by
ultrasonic spray pyrolysis at 950 �C (Fig. 2). It should be noted that, USP provides a
feasible approach for preparing shape- and size-controlled CaIn2O4 nanocrystals
using short production times that hold great potential for photocatalytic applications
and as photoluminescent materials capable of emitting white light. Moreover, the
surface/bulk defects can influence the separation of photogenerated electrone–hole
pairs on the CaIn2O4 under irradiation, and the purity of the products is high and the
composition of the powders is easily controlled.
On the other hand, pure and Al-doped ZnO nanostructured thin films were grown
at 400 �C on glass substrates by ultrasonic spray pyrolysis after 30, 60 and 120 min,
using a 0.5 M zinc acetate precursor solution (Fig. 3a) and 0.5 M zinc nitrate
precursor solution (Fig. 3b). The authors [58] found that both pure and Al-doped
ZnO nanostructured thin films show good photocatalytic activity regarding the
degradation of stearic acid under UV-A light illumination (365 nm), a behavior
which is mainly attributed to their good crystallinity and the large effective surface
area.
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Fig. 2 Schematic representation of CaIn2O4 nanocrystals prepared by ultrasonic spray pyrolysis.Reproduced and modified from Ref. [57] with kind permission from Springer Science and BusinessMedia
Fig. 3 ZnO growth mechanism using ultrasonic spray pyrolysis after 30, 60 and 120 min of sprayingtime, using a 0.5 M Zn acetate, b 0.5 M Zn nitrate precursor solution. Reproduced from Ref. [58] withkind permission from Springer Science and Business Media
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2 Perovskite Solar Cells and Quantum Dots Photovoltaics
The interest in sonochemistry as a versatile tool for materials synthesis has been
increasing, especially in the case of preparation of nanostructured materials for
energy conversion like quantum dots, solar photovoltaic cells and dye-sensitized
solar cells [59, 60]. The application of ultrasound allows to obtain a uniform shape
and highly pure nanoparticles with a narrow size distribution, which has influence
on the unique characteristics of photovoltaic devices. Additionally, ultrasound-
assisted methodologies can decrease the time of synthesis, consequently reducing
operating costs and develop effective manufacturing processes [60, 61]. Hence, it is
a promising inexpensive alternative for the production of the next generation of
solar cells such as quantum dot sensitized solar cells (QDSCs) or perovskite solar
cells (PSCs) [62]. Perovskites are a class of compounds that possess well-defined
crystal structure with formula ABX3, where, the A and B sites are engaged by
cations and the X site is occupied by the anion. Depending on the type of the ions,
they can form various perovskite crystal geometries with different features. The
cations in the lattice are able to enhance and modify the band structure, which is
particularly important from the point of view of photophysical properties [63, 64].
Thus, perovskites as an active layer in photovoltaic systems can deliver high open-
circuit voltages and lead to the harvesting of light from a broad spectrum [64].
Perovskite solar cells are able to absorb the shorter wavelengths of visible light
corresponding to photons with higher energy. Consequently, they can reach the
maximum voltage of cells adequate to the maximum electrical power that comes
from incident photons. Therefore, PSCs that stem from dye-sensitized solar cells
(DSSCs) are considered as a forerunners of emerging photovoltaic technology
[64, 65].
Perovskites first used for solar applications were documented in a seminal article
by Miyasaka and co-workers in 2009 [66], and then by Park and co-workers 2 years
later [67]. In all cases, the perovskite solar cells emerge as long-running, durable
photovoltaic devices with high efficiency, and have been increasing from 10 to
20 % in the last few years [68, 69]. Due to fact that PCSs possess an enhanced light-
harvesting system, improved charge separation process as well revised charge
transfer and charge collection [68, 69], the power conversion efficiency (PCE) of
perovskites solar cells is remarkable higher compared to the previous generation
solar cells devices [i.e., wafer-based silicon devices or thin film SCs made from
cadmium telluride (CdTe)]. The properties mentioned above can be attributed to the
special structure of these materials, which mainly reflect on their application of
photovoltaic technology as well as of other domains [70].
The next interesting possibility leading to more efficiently working solar cells is
the development of quantum dot sensitized solar cells. It is worth noting that the
energy conversion of solar cells directly depends on their structure, i.e., conductive
substrates, semiconducting layers and in particular, sensitizing molecules (dye or
quantum dots). The structure of DSSCs is similar to that of QDSCs; however, the
power conversion efficiency (PCE) is remarkably lower in the case of QDSCs.
Nevertheless, due to the relatively low cost related to using inexpensive
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semiconductors and a simple fabrication, possible approaches for improving PCE
are still under debate [61, 62, 71].
In this subsection, we would like to focus the readers’ attention on the use of
sonochemical methodology as a convenient tool for the preparation of solar cells
with higher power conversion efficiency; indicating the influence of ultrasounds on
the formation of quantum dots and perovskite solar cells, and pointing out the
current state of the art.
2.1 Sonochemical Synthesis of Perovskites and Quantum Dots
The wide range of sonochemical methods used for the synthesis of perovskite-type
oxides ABO3 have been well reviewed by Colmenares group [72]. Among the most
effective sonication-based processes, we can distinguish mainly co-precipitation
methods, sol–gel methods, and hydrothermal methods as well aerosol synthetic
techniques [e.g., the ultrasonic spray pyrolysis (USP)]. In comparison with
conventional methodologies, which are mentioned above, the USP allows for
control over size and shape of particles in a simple and more reproducible way. The
selection of optimal parameters (such as ultrasound frequency, characteristic of the
precursor solution and temperature) allows a nanomaterial with desirable properties
to be obtained. However, it is not the only effective approach for the preparation of
thin-films and ultrafine nanostructures in terms of manufacturing photovoltaic
devices. An interesting concept that involves combination of ultrasonic spray
technique and thermal evaporation process was reported by Xia et al. [73]. The
modified deposition method assisted by ultrasonic spray coating used for the
formation of CH3NH3PbI3 thin film layers enables control of morphology. The
obtained perovskite film possesses larger crystal size ([500 nm), which result in
higher carriers mobility and lower charge recombination. Thereby, uniform
coverage prevents an undesirable short circuit between the top and back surface
contacts of a solar cell, resulting in an increase in the power conversion of
photovoltaic devices [73].
CH3NH3PbI3 nanoparticles (NPs) can also be synthesized by another sonochem-
ical technique (Table 2; Entry 1) [74]. The method applied in this work involves
ultrasonic irradiation for the preparation of ultrafine nanocrystal sensitizers for solar
applications. The sonochemistry reaction is carried out in a non-aqueous solution,
leading to the generation of nanoparticles with polygonal shapes in the narrow size-
range. The sonochemical synthesis of methylammonium lead halide takes place
only in isopropanol medium, and the presence of water reduces the reaction rate
[74, 75]. Additionally, it has been shown that the morphology of the particles
strictly depends on the irradiation time [74]. The nanoparticles obtained during
10 min of reaction exhibited irregular shapes, whereas the particles achieved after
20–30 min showed an opposite trend and formed hexagonal or triangles configu-
ration [74]. Nevertheless, the shape of perovskites can be changed when the
synthesis step is preceded by ultrasonic pretreatment of the precursor solution.
Kesari and Athawale [75] demonstrated that ultrasound-assisted method arranged to
CH3NH3PbI3 synthesis allows to obtain a rod shape and a tetragonal crystal
structure. Results indicate that the preparation route has an important impact on a
59 Page 12 of 21 Top Curr Chem (Z) (2016) 374:59
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sample’s morphology and hence on its optical properties. It has been found that the
band gap of the CH3NH3PbI3 hybrid materials synthesized by ultrasonic method
(from standard 1.5 to 2.25 eV) shifted into the blue region, which can be a sign that
the materials possess features of quantum dots [74]. A similar behaviour of NaTaO3
perovskite was observed by Vazquez-Cuchillo et al. [76], when the crystalline
materials were more light sensitive in the UV-range compared to the same materials
obtained by other technique (i.e., solid-state method). On the other hand, the defects
forming on the surface due to cavitation effects can also turn to light electron
capture centres and decrease the band-gap energy, which is rather more favorable
from the photocatalytic point of view [77].
The application of ultrasound in perovskites’ synthesis allows to obtain the
required products using low temperature synthesis methods. The great number of
cavitation active sites generated under ultrasonic irradiation led to materials that
possess a small particle size and a uniform bulk [77, 78]. However, the formation
and growth of nanosheets and face-like particles depend on ultrasound as well as on
calcination conditions. The calcination temperature affects the crystallinity,
Table 2 Selected examples of sonochemical methods applied for the synthesis of active solar cells layers
Entry Ultrasound source Key parameters Remarks Refs.
Perovskites
1 The ultrasonic transducer
(frequency 20 kHz) was
operated at an amplitude of
60 %
Time of irradiation
increased from 10
to 30 min
The CH3NH3PbI3 nanoparticles
of polygonal shapes in the
size range of 10–40 nm were
obtained until 20 min
reaction, whereas the shorter
time of irradiation resulted in
non-uniform shapes
[74]
2 A multiwave ultrasonic
generator equipped with
titanium oscillator
(12.5 mm) operating at
20 kHz
Power of ultrasound
was adjusted in
the range from 50
to 70 W
The particle size of CuInS2 was
about 100 nm when the power
of ultrasound was equal to
50 W—increasing the power
led to formation of uniform
particles with size of 60 nm
[81]
3 A sonication system operated
at 59 kHz and a power
output of 99 W
Concentration of
precursor and
sonication time
(various variants)
The desired kesterite phase of
CuZnSnS2 was obtain using
longer time of irradiation and
higher precursor
concentration
[82]
Quantum dots
4 A ultrasound probe (frequency
20 kHz, 130 W/cm2)
operated at 50 % amplitude
The effect of
ultrasound on the
reduction process
The reduction process of
tellurium does not exceed
15 min and allows to obtain
CdTe QDs with the band gap
value (2.3 eV)
[85]
5 Ultrasound horn with 1.9 cm
diameter (20 kHz, output
acoustic power 45.5 W)
The sonication time
(0–45 min) and
temperature
(40–60 �C)
The CdS nanoparticles have
uniform spherical
morphology with size around
2 nm (60 �C after 45 min of
irradiation).
[86]
Top Curr Chem (Z) (2016) 374:59 Page 13 of 21 59
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microstructure, optical and dielectric features of the samples. Consequently, the
particles that were synthesized under the same ultrasonic conditions and calcined at
different temperatures exhibited diverse properties as proved by various researcher
groups [79, 80]. Interesting is the fact that contrary to previous studies, Wirunchit
et al. [80] presented a facile sonochemical synthesis of perovskite oxides without
any sintering and calcination step (Fig. 4). In this case, the formation of
nanoparticles with a spherical morphology and a narrow particle size distribution
occurs through the sonocrystallization process, whereas the effect of ultrasonic
irradiation generates and promotes the nucleation as well as inhibits or delays the
crystal growth process [80]. Furthermore, the crystal size and morphology of
nanostructures also depend on the power energy of ultrasound (Table 2; Entry 2).
Results presented by Amiri et al. indicate that increasing the power of ultrasound
causes the production of uniform CuInS2 nanoparticles with small size (*60 nm),
whereas using the lower ultrasonic energy led to the creation of bigger aggregated
particles with a lump-like structure [81].
The one-step sonochemical method proposed by Liu et al. [82] can be applied for
the production of Cu2ZnSnS4 perovskites (Table 2; Entry 3). By this way, the
propitious shape of nanoparticles can be obtained by facile and rapid synthesis
through the selection of an optimal precursor’s concentration as well the above-
mentioned sonication time [82]. However, in the event of the production of high
quality inks for solar-cells fabrication, a more suitable methodology seems to be the
ultrasound-assisted microwave solvothermal method. Unlike the others, the
nucleation–dissolution–recrystallization mechanism allows to obtain Cu2ZnSn4
Fig. 4 Schematic diagrams illustrating formation of the crystal growth mechanism. Reproduced from[80] with permission of The Royal Society of Chemistry
59 Page 14 of 21 Top Curr Chem (Z) (2016) 374:59
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nanosized particles with hexagonal prisms structure, due to fact that the synergistic
effect of ultrasound and microwave plays a key role in the growth process of
wurtzite phase [83, 84].
Ultrasound-assisted processes are also used to prepare quantum dot particles
(QDs) for solar cells application. The simple and fast sonochemical methodology
provides monodispersed CdTe QDs with a strong quantum confinement regime
(Table 2; Entry 4). The application of ultrasound allows one to control the
morphology and reduce the surface defects of quantum dots. As a result, this
allowed the production of nanoparticles with only one fluorescence band, which is
beneficial in terms of photovoltaic devices [85]. Furthermore, it is clearly shown
that the application of ultrasound, in particular the sonication time, has an influence
on the particle size distribution and the growth kinetics (Table 2; Entry 5) [86].
Results indicate that the increase of particle size at longer time is attributed to the
diffusion-limited coarsening process, which is accelerated by cavitation. Thus, the
effect of ultrasound irradiation causes that the synthesis of QDs nanoparticles occurs
in shorter time using low temperature methodology such as the micro-emulsion
method [86]. Additionally, sonochemically synthesized QDs possess better optical
properties and are able to produce a large amount of reactive oxygen species (ROS),
which may influence a further course of the photochemical processes [87].
2.2 Sonochemical Solar Cells Fabrication
Sonochemical methodology can be used as the first step toward the synthesis of
photoactive layers or as the second step for the preparation of solar cells devices.
The wide range of ultrasound parameters (i.e., ultrasonic power, frequency and
time) have a positive influence on the size, morphology, structure or the surface
area, and in consequence on optical as well as electrical properties of photovoltaic
devices [81]. It was proven that the ultrasound irradiation accelerates the nucleation
process, and promotes ionic and mass diffusion, which permit the synthesis of
structures with desirable features in terms of the photovoltaic application [88]. For
this reason, ultrasounds play a crucial role in the preparation of solar cells structures
and cells’ components.
The photoactive layers can consist of perovskite nanoparticles, quantum dots or
hierarchical hollow spheres which can improve the efficiency of dye-sensitized solar
cells (DSSCs), or quantum dots solar cells [89]. The DSSCs type of solar cells
exhibited significant better parameters (i.e., a short circuit current density, open
circuit voltage and fill factor) in comparison to standard nanoparticle photoelec-
trode, consequently increasing the power conversion efficiency and photocatalytic
performance [90]. The hierarchical structures (HSs) (mainly ZnO, TiO2, SnO2)
synthesized through the sonochemistry methodology demonstrate required proper-
ties caused by their nano/micro combined architectures. They enable fast electron
transport due to an ideal network created by nanosheets connection. Additionally,
HSs possess a large surface area, which improves the accessibility for incident
photons [88, 91]. Furthermore, dye-sensitized solar cells based on hierarchical
structures are able to enhance the light harvesting capability and improve the
Top Curr Chem (Z) (2016) 374:59 Page 15 of 21 59
123
efficiency of SCs [91]. Using ultrasound treatment technique (UST), the change of
the solar power efficiency can be improved up to 50 % [92].
Among many sonochemical methods used for the fabrication of solar cells, we
can spot the ultrasonic spray coating method (USP). The USP allows one to deposit
uniform perovskite thin films on glass substrates (Fig. 5) [93, 94]. The formation of
dense films having a surface coverage above 85 % guarantees maximum device
efficiency. For this reason, the USP coating leads to an increase of the power
conversion efficiencies of perovskite-based photovoltaics devices [94]. Concur-
rently, ultrasonic spray coating with various rates provides the quick optimization of
thickness, precursor ratios and resulting pinhole-free layer crystallinity [95].
Furthermore, USP provides high-performance flexible perovskite solar cells by
using a combination of ultrasonic spray-coating and low thermal budget photonic
curing [94].
Fig. 5 Schematic of pumped ultrasonic spray coating for perovskite precursor deposition. Reproducedfrom [94] with permission of The Royal Society of Chemistry
Fig. 6 Scheme of the automated manufacturing of SVADC process for solar cells fabrication.Reproduced and modified from Ref. [96] with kind permission from Springer Science and BusinessMedia
59 Page 16 of 21 Top Curr Chem (Z) (2016) 374:59
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As the next promising tool for the fabrication of micro-thin as well as nano-thin
films, the substrate vibration-assisted drop casting method (SVADC) has emerged.
Due to fact that the SVADC is scalable casting methodology, it can be used to
prepare an array of thin-film solar cells, perovskite, and quantum-dot solar cells or
other thin-film devices through an automated fabrication process (Fig. 6). The same
manufacturing process allows several layers of a thin-film solar cell to be deposited.
It is worth to note that the solution properties, surface wettability, impingement
conditions as well as substrate vibration have influence on the maximum effective
and uniform surface of solar cells. Due to the fact that this method allows one to
obtain a simple structure (possessing efficiency over 3 %) without requiring an
optimization process and the application of expensive materials or treatments, the
SVADC might replace the old generation techniques of ultrasound-assisted methods
used for the fabrication of silicon wafers [96].
Acknowledgments Prof. Dr. Juan C. Colmenares would like to thank the National Science Centre
(Poland) for support within the project Sonata Bis Nr. 2015/18/E/ST5/00306. Paweł Lisowski would like
also to thank the National Science Centre (NCN) in Poland for the research project 2015/17/N/ST5/
03330.
Open Access This article is distributed under the terms of the Creative Commons Attribution 4.0
International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, dis-
tribution, and reproduction in any medium, provided you give appropriate credit to the original
author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were
made.
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